linux-kernel - Re: [RFC PATCH tip 0/5] tracing filters with BPF

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Date:	Thu, 5 Dec 2013 21:16:43 -0800
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Tom Zanussi <tom.zanussi@...ux.intel.com>,
	Jovi Zhangwei <jovi.zhangwei@...il.com>,
	Eric Dumazet <edumazet@...gle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH tip 0/5] tracing filters with BPF

On Thu, Dec 5, 2013 at 5:20 PM, Andi Kleen <andi@...stfloor.org> wrote:
>> the difference is bigger now: 484-145 vs 185-145
>
> This is a obvious improvement, but imho not big enough to be extremely
> compelling (< cost 1-2 cache misses, no orders of magnitude improvements
> that would justify a lot of code)

hmm. we're comparing against ktap here…
which has 5x more kernel code and 8x slower in this test...

> Your code requires a compiler, so from my perspective it
> wouldn't be a lot easier or faster to use than just changing
> the code directly and recompile.
>
> The users want something simple too that shields them from
> having to learn all the internals. They don't want to recompile.
> As far as I can tell your code is a bit too low level for that,
> and the requirement for the compiler may also scare them.
>
> Where exactly does it fit?

the goal is to have llvm compiler next to perf, wrapped in a user friendly way.

compiling small filter vs recompiling full kernel…
inserting into live kernel vs rebooting …
not sure how you're saying it's equivalent.

In my kernel debugging experience current tools (tracing, systemtap)
were rarely enough.
I always had to add my own printks through the code, recompile and reboot.
Often just to see that it's not the place where I want to print things
or it's too verbose.
Then I would adjust printks, recompile and reboot again.
That was slow and tedious, since I would be crashing things from time to time
just because skb doesn't always have a valid dev or I made a typo.
For debugging I do really need something quick and dirty that lets me
add my own printk
of whatever structs I want anywhere in the kernel without crashing it.
That's exactly what bpf tracing filters do.
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